Platinum complexes of diazo ligands. Studies of regioselective aromatic ring amination, oxidative halogen addition and reductive halogen elimination reactions

A review on the chemistry of novel platinum chelates based on azo-azomethine ligands

Numerous platinum group metals (PGMs) complexes contain azo-azomethine-based ligands. Azo-azomethine ligands are N-donor ligands that have extended conjugated π-bonded systems and both azo (–N=N–) and aldimine (–C=N–) functions in their structure. Plenty of platinum (Pt) complexes with azo-imine ligands have been prepared and characterized. Various multidentate azo-imine ligands coordinated with different platinum metal substrates afforded structurally diverse platinum chelates. Nonetheless, many azo-imine-based platinum complexes demonstrated a wide range of biological activities, photo-switchable properties, and redox activities. The review encompasses a general overview of platinum complexes with versatile azo-azomethine ligands, their synthetic protocol, spectroscopic and structural features, chemical reactivity, and multipurpose applications in different areas.

Photoactive Platinum(II) Azopyridine Complexes†

Platinum(II) complexes containing the strong π-acceptor N,N-chelating ligand phenylazopyridine (Ph-azpy) [Pt(p-R-Ph-azpy)X₂ ], R = H, NMe₂ or OH, X = Cl or N₃ , have been synthesized and characterized to explore the effects of monodentate ligands and phenyl substituents on their absorption spectra and photoactivation. Time-dependent density functional theory calculations showed that the complexes have a low-lying unoccupied orbital with strong σ-antibonding character toward the majority of the coordination bonds. The UV-visible absorption bands were assigned as mainly ligand-centered or metal-to-ligand charge-transfer transitions, with strong contributions from the chlorido and azido groups. In complexes with substituted Ph-azpy ligands, σ-donation from NMe₂ and OH/O^- groups results in a redshift of the main absorption bands compared with unsubstituted Ph-azpy complexes. The diazido complexes are photoactive in solution upon irradiation with either UVA or visible light for R = H or NMe₂ , or UVA only when R = OH/O^- . Intriguingly, the phenolate group of the latter complex undergoes very slow protonation in solution. Biological screening was limited by poor solubility; however, initial tests showed that the phenolato diazido complex is rapidly taken up into the nuclei of HaCaT keratinocytes, which are stained intensely blue, and its cytotoxicity is increased upon irradiation with UVA light.

Versatile Coordination of Azocarboxamides: Redox-Triggered Change of the Chelating Binding Pocket in Ruthenium Complexes

Azocarboxamides occupy a special place among azo ligands owing to their versatility for metal coordination. Herein ruthenium complexes with two different azocarboxamide ligands that differ in the presence (or not) of a coordinating pyridyl heterocycle are presented. By making full use of the O,N(amide), N(azo), and N(pyridyl) coordinating sites, the first diruthenium complex that is bridged by an azo ligand containing two different binding pockets was obtained. Moreover, it was conclusively proven that, in the mononuclear complexes, oxidation at the ruthenium center leads to a complete change of coordination at the chelating binding pocket. The complexes were characterized by NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction. Additionally, the mechanism of the aforementioned redox-triggered change in the chelating binding pocket and the electronic structures of all the complexes were investigated by a combination of electrochemistry, UV/Vis/NIR/EPR spectroelectrochemistry, and DFT calculations. This is first instance in which a redox-driven change in the complete chelating binding pocket has been observed in a ruthenium complex as well as with azo-based ligands. These results thus show the potential of these versatile azocarboxamide ligands to act as redox-driven switches with possible relevance to electrocatalysis.

Hydrogen bonding and fluorous weak interactions in the non-isomorphous {4,4'-bis[(2,2,3,3-tetrafluoropropoxy)methyl]-2,2'-bipyridine-κ2N,N'}dibromidopalladium and -platinum complexes

The polyfluorinated title compounds, [MBr₂(C₁₈H₁₆F₈N₂O₂)] or [4,4'-(HCF₂CF₂CH₂OCH₂)₂-2,2'-bpy]MBr₂, (1) (M = Pd and bpy is bipyridine) and (2) (M = Pt), have -CH_(α)2OCH_(β)2CF₂CF₂H side chains with methylene H-atom donors at the α and β sites, and methine H-atom donors at the terminal sites, in addition to aromatic H-atom donors. In contrast to the original expectation of isomorphous structures, (1) crystallizes in the space group C2/c and (2) in P2₁/n, with similar unit-cell volumes and Z = 4. The asymmetric unit of (1) is one half of the molecule, which resides on a crystallographic twofold axis. Both (1) and (2) display stacking of the molecules, indicating a planar (bpy)MBr₂ skeleton in each case. The structure of (1) exhibits columns with C-H_(β)...Br hydrogen bonds between consecutive layers which conforms to a static (β,β) linkage between layers. In the molecular plane, (1) shows double C-H_(α)...Br hydrogen bonds self-repeating along the b axis, the planar molecules being connected into infinite belts. Compound (2) has no crystallographic symmetry and forms π-dimer pairs as supermolecules, which then stack parallel to the a axis. The π-dimer-pair supermolecules exhibit (Pt-)Br...Br(-Pt) contacts [3.6937 (7) Å] to neighbouring π-dimer pairs crosslinking the columns. The structure of (2) reveals many C-H...F(-C) interactions between F atoms and aromatic C-H groups, in addition to those between F atoms and methylene C-H groups.

Ferromagnetic coupling mediated by Co⋯π non-covalent contacts in a pentacoordinate Co(ii) compound showing field-induced slow relaxation of magnetization

A mononuclear pentacoordinate Co(ii) complex exhibits weak ferromagnetic interactions through a Co⋯π superexchange pathway.

New mixed ligand palladium(II) complexes based on the antiepileptic drug sodium valproate and bioactive nitrogen-donor ligands: synthesis, structural characterization, binding interactions with DNA and BSA, in vitro cytotoxicity studies and DFT calculations

The complexes [Pd(valp)2(imidazole)2] (1), [Pd(valp)2(pyrazine)2] (2) (valp is sodium valproate) have been synthesized and characterized using IR, (1)H NMR, (13)C{(1)H} NMR and UV-Vis spectrometry. The interaction of complexes with CT-DNA has been investigated using spectroscopic tools and viscosity measurement. In each case, the association constant (Kb) was deduced from the absorption spectral study and the number of binding sites (n) and the binding constant (K) were calculated from relevant fluorescence quenching data. As a result, a non-covalent interaction between the metal complex and DNA was suggested, which could be assigned to an intercalative binding. In addition, the interaction of 1 and 2 was ventured with bovine serum albumin (BSA) with the help of absorption and fluorescence spectroscopy measurements. Through these techniques, the apparent association constant (Kapp) and the binding constant (K) could be calculated for each complex. Evaluation of cytotoxic activity of the complexes against four different cancer cell lines proved that the complexes exhibited cytotoxic specificity and significant cancer cell inhibitory rate. Moreover, density functional theory (DFT) calculations were employed to provide more evidence about the observed data. The majority of trans isomers were supported not only by energies, but also by the similarity of its calculated IR frequencies, UV adsorptions and NMR chemical shifts to the experimental values.

Recent advances on the chemistry of transition metal complexes of 2-(arylazo)pyridines and its arylamino derivatives

Recent advancement on the redox properties of a selection of transition metal complexes of the azoaromatic ligands: bidentate L(1) [2-(arylazo)pyridine] and tridentate HL(2) [2-(aminoarylphenylazo)pyridine] are described and compared. Due to the presence of a low lying azo-centered π*-orbital, these azoaromatic ligands may exist in multiple valent states. The coordination chemistry of the L(1) ligands was thoroughly studied during the 1980s. These complexes undergo facile reduction in solution at low accessible potentials. One electron reduced azo-complexes, though known for a long time to occur in solution, have only recently been isolated in a crystalline state. New synthetic protocols for the synthesis of metal-bound azo-radical complexes have been developed. Low-valent metal complexes such as metal carbonyls have been found to be excellent starting materials for this purpose. In a few selected cases, syntheses of these complexes were also achieved from very high valent metal oxides using triphenylphosphine as both a reducing and oxo-abstracting agent. Issues related to the ambiguities of the electronic structures in the azo-metal complexes have been discussed considering bond parameters, redox and spectral properties. Unusual redox events such as RIET (Redox-Induced Electron Transfer) phenomena in a few systems have been elaborated and compared with the known example. Novel examples of N=N bond cleavage reactions via four-electron reduction and subsequent C-N bond formation in metal-bound coordinated ligands have been noted.

Platinum(IV)-kappa(3)-terpyridine complexes: synthesis with spectroscopic and structural characterization

Herein we report the first structural characterization of a Pt(iv)-kappa(3)-terpyridine species, [Pt((t)Bu(3)tpy)Cl(3)][AuCl(2)], which can be prepared by oxidation with [AuCl(4)](-). The syntheses and characterization of [Pt((t)Bu(3)tpy)Cl(3)]Cl ((t)Bu(3)tpy is 4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine), as well as the platinum(ii) complexes [Pt((t)Bu(3)tpy)Cl]Cl and [Pt((t)Bu(3)tpy)Cl][AuCl(2)] are reported.

First metallamacrocyclic complexes of Pt(iv) with 3,3,3′,3′-tetraalkyl-1,1′-phenylenedicarbonylbis(thioureas): synthesis by direct or electrolytic oxidative addition of I2, Br2 and Cl2

Elemental I(2) and Br(2) cleanly react with the 3:3 Pt(ii) metallamacrocycle of 3,3,3',3'-tetra(n-butyl)-1,1'-terephthaloylbis(thiourea)(cis-[Pt(II)(3)(L(p)(1)-S,O)(3)]3), in chloroform at room temperature, to yield oxidative addition products; (195)Pt NMR studies reveal that a stepwise oxidative addition readily occurs to each of the Pt(ii) centres in the metallamacrocycle to yield the mixed valence species cis-[Pt(II)(2)Pt(IV)I(2)(L(p)(1)-S,O)(3)] and cis-[Pt(II)Pt(IV)(2)I(4)(L(p)(1)-S,O)(3)], and the fully oxidised cis-[Pt(IV)(3)I(6)(L(p)(1)-S,O)(3)] in solution, depending on the mole ratio I(2):3. Similar results are obtained on treatment of solutions of 3 with elemental Br(2). Treatment of the corresponding 2:2 Pt(ii) complex of 3,3,3',3'-tetraethyl-1,1'-isophthaloylbis(thiourea)(cis-[Pt(II)(2)(L(m)(1)-S,O)(2)]4) with iodine, results in facile oxidative addition to yield cis-[Pt(IV)(2)(L(m)(1)-S,O)(2)I(4)], with a trans-Pt(iv)-iodo arrangement. Molecules in the crystal structure of 5 have their trans-Pt(iv)-iodo axes essentially aligned, with very close intermolecular iodide contacts (3.775(1)A), resulting in chains of weakly bound metallamacrocycles in the solid. An alternative electrolytic synthesis method, using a simple two-compartment glass cell containing 4 and a chosen halide salt in dichloromethane, led to the formation of cis-[Pt(IV)(2)(L(m)(1)-S,O)(2)Br(4)] 6 and cis-[Pt(IV)(2)(L(m)(1)-S,O)(2)Cl(4)] 7, completing characterization of a series of first-reported trans-Pt(iv)-X (X=I, Br, Cl) metallamacrocyclic complexes.